Compounded resins comprising a synthetic resin and a finely divided organic or inorganic material admixed therewith may be subjected to injection molding usually by preparing pellets with a kneader and then feeding the pellets to a separate injection molding machine for melting and injection molding therein to obtain a compounded resin product. Alternatively the materials may be kneaded and injection-molded by a continuous process with use of an injection molding machine having a small kneader attached to the feed inlet of the machine without prior pelletizing of the materials. However, such conventional methods have the following drawbacks. The former method involves the expenditures of of considerable heat energy and requires much labor since the resin is first melted in the kneader with application of heat, and, after cooling, is again heated and remelted for injection molding. On the other hand, the latter method which is adapted for a continuous process effects a savings in energy and labor but requires expensive apparatus. Because of this the former method has found wide use.
Extrusion molding machines have heretofore been proposed in which the shape of the screw is modified in various ways for the kneading operation to simplify the construction of the machine. Many of these proposals, however, are adapted to produce a shearing action only and are, therefore, unsatisfactory for kneading compounded resin compositions. Screws designed for compression in addition to shearing are complex in shape and difficult to make. They also have the drawback of permitting stagnation of the resin.